1. Field of the Invention
The invention relates to an ophthalmological illuminating device and method, in particular it relates to an illuminating device of a surgical microscope for observing an examined eye.
2. Description of Related Art
Surgical microscope illumination devices provide coaxial illumination methods and oblique illumination methods. In the coaxial illumination method, the illumination optical axis and the observation optical axis of the microscope coincide so that the illumination light is not blocked by the edges of the illuminated viewing area. In the oblique illumination method, a large angle between the illumination optical axis and the observation optical axis of the microscope enables three-dimensional observation based on shadows generated by the illumination light.
The coaxial illumination method allows the illumination light to reach deeply in the viewing areas. The depth depends on the extent that the illumination optical axis coincides with the observation optical axis. However, damage to the macula lutea of the retina may occur by direct illumination.
In the coaxial illumination method, an illumination angle between the illumination optical axis and the observation optical axis of the microscope is changed by advancing or withdrawing a prism along the illumination light path. The illumination angle is also changed by moving the reflective mirror on the illumination optical axis.
The oblique illumination method is used when the coincidence of the illumination optical axis and the observation optical axis is not necessary. In general, the oblique illumination method is driven by a different power source than that of the coaxial illumination method. Thus, both an oblique illumination system and a coaxial illumination system are required for a surgical microscope. Accordingly, the size and cost of the illumination device are large and reduction in the size and cost of the illumination device are desired.
In order to reduce the size and cost of the illumination device, a reflective mirror is provided in a conventional illuminating device. The reflective mirror is moved into the illumination light path for oblique illumination and the reflective mirror is withdrawn from the illumination light path for coaxial illumination.
In the coaxial illumination method, when the illumination angle is changed by moving a prism into and out of the illumination light path, only predetermined illumination angles can be obtained. Consequently, intense light shining on the macula lutea of the retina may be unavoidable.
A continuous change in the illumination angle is achieved, when the illumination angle is changed by a reflective mirror in the coaxial illumination method. However, because the illumination light always has a fixed light quantity, excessive light shines on the macula lutea of the retina when the illumination angle is small.
Furthermore, when the reflective mirror is used to change between oblique illumination and coaxial illumination, concurrent use of both the coaxial illumination and the oblique illumination is impossible. Consequently, when using coaxial illumination with a narrow illumination field radius, it is impossible to illuminate the wide portions surrounding the perimeter of the viewing area.